Sub-picosecond to Sub-nanosecond Vibrational Energy Transfer Dynamics in Pentaerythritol Tetranitrate

Abstract

The time scale associated with shock-induced detonation is a key property of energetic materials that remains poorly understood. Herein, we test aspects of one potential mechanism, the phonon up-pumping mechanism, where shock compression excites lattice phonon modes, transferring energy to intramolecular vibrations leading to chemical bond cleavage and reaction. Using ultrafast infrared pump−probe spectroscopy on pentaerythritol tetranitrate (PETN), we reveal sub-picosecond vibrational energy transfer (VET) from the photoexcited band at 1660 cm −1 into every other infrared-active mode in the probed frequency range 800−1800 cm −1 . Energy transfer processes remain incomplete at 150 ps. Computational predictions from density functional theory are used in tandem to elucidate VET pathways in PETN.

Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-Triazine (RDX) Thin Films

“…43 The apparatus used for ultrafast broadband infrared spectroscopy has been previously described in detail. 13,44 Briefly, the ~50 fs, 780 nm output from a commercial Ti:Sapphire laser is split into "pump" and "probe" arms, where the pump photoexcites the sample and the probe measures the pump-induced response. The pump arm of the laser output is first sent into a commercial optical parametric amplifier (OPA), whose signal and idler outputs are difference frequency mixed (DFM) and sent through a bandpass filter to generate tunable, narrowband midinfrared pulses that excite specific infrared transitions in the material.…”

“…While there is rich literature investigating shockwave-induced processes in reactive and inert solids (see Refs. [7][8][9][10] for recent examples), little experimental work exists [11][12][13][14] on vibrational dynamics in solid, neat EMs that shed light on VET through strongly coupled, delocalized vibrational modes. Much of the existing VET research is performed in EM mimics or solvated energetics (see Refs.…”

“…Given the broad, far-to mid-infrared, frequency range of vibrations in molecular crystals, VET investigations require ultrafast infrared spectroscopy to be coupled with broadband infrared pulses to derive a global view of VET dynamics. A recent study by our team used ultrafast broadband infrared spectroscopy and density functional theory to investigate VET in a pentaerythritol tetranitrate (PETN-I) thin film, revealing dynamics ranging from sub-picosecond to hundreds of picosecond timescales, and suggesting that VET is incomplete even up to 150 ps, 13 an unexpected finding given the typical timescales assumed for statistical energy partitioning. 18 Unfortunately, due to the broad, overlapping vibrational transitions in PETN, we were unable to determine whether the long-lived VET process was due to persistent excitation of certain highfrequency vibrations or continued evolution of a non-thermal distribution of low frequency lattice modes that, in turn, perturbed 6 the measured high-frequency vibrations.…”

Vibrational energy redistribution in crystalline nitromethane simulated by ab initio molecular dynamics

“…Vibrational energy redistribution (VER) or vibrational energy transfer (VET) has a signicant impact on the physicochemical dynamics of energetic materials (EMs), such as their initiation to detonation and impact sensitivities. [1][2][3][4][5] To elucide the VET paths and the underlying mechanisms, nitromethane (NM) as the simplest one of the nitro EMs has been of great interest for many decades.…”

“…16 Theoretically, there have been plenty of studies on the physical and chemical processes of NM, which focus on the early physical events and reaction chemistry under thermal or shock loading. 5,[17][18][19][20][21] For example, Kabadi and Rice studied the VER in liquid NM aer exciting CH 3 stretching vibrations by nonequilibrium molecular dynamics (NEMD) based on classical force eld. 17 The results show the multistage VER processes at different rates agree qualitatively with the earlier measurement by Dlott's group.…”